X-ray CT (Computed Tomography) is a technology wherein an image representing information of an object is formed by reconstructing projection data obtained by scanning the object with X-ray beams.
As an application of X-ray CT, there is technology that identifies a substance included in an object. In this technology, since it is difficult to determine a substance based on an image obtained by using X-ray beams of a single tube voltage, a method recently has attracted attention that involves taking individual scans with two X-ray beams of different energy levels by using two different tube voltages. This method is known as “dual energy CT.”
Non-patent Document 1 describes technology wherein two images are formed by applying two different tube voltages, and a substance is identified by using ratios of the CT numbers of these images. It should be noted that any of the matters described in Non-patent Document 1 is made a reference as a part of the present specification.
Furthermore, Patent Document 1 describes technology wherein an image (reference substance image) is formed that expresses the distribution of various reference substances, through expressing the linear attenuation coefficient shown in each projection data obtained from the use of two different tube voltages as the linear combination of the linear attenuation coefficients of two reference substances (for example, water and bone). Furthermore, Patent Document 1 also describes methods for the formation of an effective atomic number image, a density image and a monochromatic X-ray image through combining these reference substance images. It should be noted that any of the matters described in Patent Document 1 is made a reference as a part of the present specification.